In the automotive industry polymeric fuel containers are being used because they can be manufactured in a cost-efficient manner, are of high mechanical stability and readily deformable in accidents, and greatly inhibit hydrocarbon permeation. The best results as to the overall qualities of the polymeric fuel containers have been obtained by a six-layer so-called COEX-structure. This is a multi-layer system manufactured in a single process (co-extrusion) and including two layers of high density polyethylene (HDPE) which enclose a barrier-layer of an ethylene vinyl alcohol copolymer (EVOH) and a layer of treated, recycled or “re-grind” plastic material.
The EVOH-layer, which is not directly connectable to the HDPE, has an adhesive layer provided on both of its sides for connection to the adjacent layers so that the total structure comprises six layers. The layer of recycled or re-grind material is of a thickness which is about 35 to 45% of the total thickness of the fuel container wall and consists of a mixture of scrap materials resulting from the manufacturing of containers, i.e. it is both of HDPE and of EVOH. While HDPE is cheap and has good mechanical characteristics, it is a poor barrier against permeation of hydrocarbons. This is why the relatively thin EVOH-layer is used, which while being expensive, is an excellent barrier against permeation of hydrocarbons. Additional techniques include fluorinating the polyethylene to make it inherently more impermeable to fuel vapors.
Presently, the State of California generally has the most stringent requirements for the reduction of total vehicle hydrocarbon emissions. As a general rule the other states in the U.S. and many other countries will adopt the Californian regulations after some time. Under the provisions of such future regulations the level of total vehicle hydrocarbon permeation must not exceed 0.5 g per day. To achieve this level, it has been estimated that the hydrocarbon emissions from the vehicle fuel system must not be more than 150 mg per day, which would result in a static permeation of less than 55 mg per day when production and durability parameters are considered. However, the fuel container is only a part of the total fuel system, and further estimates have shown that permeation through the container wall should not exceed 5 mg per day in order to meet the above requirements. The above described typical six-layer COEX-structure, however, only provides permeation levels of about 20 mg or less per day. One possibility to improve the performance of the six-layer COEX-structure would be to increase the thickness of the EVOH-layer from about 150 micrometers to about 1.0 mm. Apart from substantially increased costs this would cause production problems and deteriorate the mechanical properties of the fuel container because EVOH has relatively poor impact resistance. This structure also has a permeation window which results when the two wall halves are welded together by pinching under heat.
U.S. Pat. No. 6,719,163 teaches eliminating the 6 layer co-extruded structure and its associated permeation window, with a multilayer structure containing two separately manufactured halves with the barrier layer on the outside of the structure. U.S. Pat. No. 6,719,163 teaches the layers of its fuel tank be made of high density polyethylene and a compound impermeable to fuel such as ethyl vinyl alcohol (EVOH).
European Patent EP 742 236 describes petrol tanks consisting of five layers which are, respectively: high density polyethylene (HDPE); a binder; a polyamide (PA) or a copolymer containing ethylene units and vinyl alcohol units (EVOH); a binder; and HDPE.
A sixth layer can be added between one of the layers of binder and one of the HDPE layers. This sixth layer consists of manufacturing scraps following molding of the tanks, and to a much smaller extent of non-compliant tanks. These scraps and non-compliant tanks are then ground until granules are obtained. This ground material is then re-melted and extruded directly at the tank co-extrusion plant. This ground material may also be melted and re-granulated by means of an extruding machine such as a twin-screw extruder, before being reused.
According to one variant, the recycled product can be mixed with the HDPE from the two extreme layers of the tank. It is possible, for example, to mix the granules of recycled product with granules of virgin HDPE of these two layers. It is also possible to use any combination of these two recyclings. The content of recycled material can represent up to 50% of the total weight of the tank.
European Patent EP 731 308 describes a tube comprising an inner layer comprising a mixture of polyamide and of polyolefin with a polyamide matrix and an outer layer comprising a polyamide. These tubes based on polyamide are useful for transporting petrol and more particularly for bringing the petrol from the motor vehicle tank to the motor and also, but in larger diameter, for transporting hydrocarbons in service stations between the distribution pumps and the underground storage tanks
According to another form of the tube, a layer of a polymer comprising ethylene units and vinyl alcohol units (EVOH) can be placed between the inner and outer layers. The structure: inner layer/EVOH/binder/outer layer is advantageously used.
The tanks described in EP 742 236 which do not have the barrier layer in direct contact with the petrol do admittedly have barrier properties, but they are not sufficient when very low petrol losses are desired. EP 731 308 describes tubes whose outer layer is made of polyamide and the barrier layer is in direct contact with the petrol, wherein the layer made of polyamide is necessary for the mechanical strength of the assembly.
United States Patent Application No. 20030198768 proposes a plastic fuel tank having a multilayer wall structure, wherein a barrier layer forms an exposed face of the wall and preferably is in direct contact with the fuel contained therein. The barrier layer of the structures of the invention constitutes one of the exposed faces of the structure, i.e. it is not an interior layer of the wall structure. Fuel tank structures embodying the invention have walls with HDPE/barrier layer or HDPE/binder/barrier layer, in which “HDPE” denotes high density polyethylene.
The preferred fuel tank of United States Patent Application No. 20030198768, comprises successively: a first layer of high density polyethylene (HDPE), a layer of binder, a second layer of EVOH or of a mixture based on EVOH, and optionally a third layer of polyamide (A) or a mixture of polyamide (A) and polyolefin (B).
The polyamides proposed in United States Patent Application No. 20030198768 are those in which the term “polyamide” means the following products of condensation of one or more amino acids, such as aminocaproic acid, 7-aminoheptanoic acid, 11-aminoundecanoic acid and 12-aminododecanoic acid of one or more lactams such as caprolactam, oenantholactam and lauryllactam; and of one or more salts or mixtures of diamines such as hexamethylenediamine, dodecamethylenediamine, meta-xylylenediamine, bis(p-aminocyclohexyl)methane and trimethylhexamethylenediamine with diacids such as isophthalic acid, terephthalic acid, adipic acid, azelaic acid, suberic acid, sebacic acid and dodecanedicarboxylic acid.
As examples of polyamides, United States Patent Application No. 20030198768 mentions PA 6 and PA 6-6 and copolyamides. United States Patent Application No. 20030198768 also mentions copolyamides resulting from the condensation of at least two α,{acute over (ω)}-aminocarboxylic acids or of two lactams or of one lactam and one α,{acute over (ω)}-aminocarboxylic acid. Also included in United States Patent Application No. 20030198768 are the copolyamides resulting from the condensation of at least one α,{acute over (ω)}-aminocarboxylic acid (or a lactam), at least one diamine and at least one dicarboxylic acid.
United States Patent Application No. 20030198768 prefers lactams containing from 3 to 12 carbon atoms on the main ring and which can be substituted. Examples of such lactams are β,β-dimethylpropiolactam, α,α-dimethylpropiolactam, amylolactam, caprolactam, capryllactam and lauryllactam.
Aminoundecanoic acid and aminododecanoic acid are examples of α,{acute over (ω)}-aminocarboxylic acids and adipic acid, sebacic acid, isophthalic acid, butanedioic acid, 1,4-cyclohexanedicarboxylic acid, terephthalic acid, sodium or lithium salts of sulphoisophthalic acid, dimerized fatty acids (these dimerized fatty acids have a dimer content of at least 98% and are preferably hydrogenated) and dodecanedioic acid HOOC—(CH2)10—COOH are examples of dicarboxylic acids.
The diamine noted in United States Patent Application No. 20030198768 can be an aliphatic diamine containing from 6 to 12 atoms and can be arylic and/or saturated cyclic. Hexamethylenediamine, piperazine, tetramethylenediamine, octamethylenediamine, decamethylenediamine, dodecamethylenediamine, 1,5-diaminohexane, 2,2,4-trimethyl-1,6-diaminohexane, diaminepolyols, isophoronediamine (IPD), methylpentamethylenediamine (MPDM), bis(aminocyclohexyl)methane (BACM) and bis(3-methyl-4-aminocyclohexyl)methane (BMACM) are examples of such diamines.
Copolymers of caprolactam and of lauryllactam (PA 6/12), copolymers of caprolactam, of adipic acid and of hexamethylenediamine (PA 6/6-6), copolymers of caprolactam, of lauryllactam, of adipic acid and of hexamethylenediamine (PA 6/12/6-6), copolymers of caprolactam, of lauryllactam, of 11-aminoundecanoic acid, of azelaic acid and of hexamethylenediamine (PA 6/6-9/11/12), copolymers of caprolactam, of lauryllactam, of 11-aminoundecanoic acid, of adipic acid and of hexamethylenediamine (PA 6/6-6/11/12) and copolymers of lauryllactam, of azelaic acid and of hexamethylenediamine (PA 6-9/12) are examples of copolyamides. The problem with the use of most polyamides is high shrinkage from the mold. Polyamides also present a problem of high moisture absorption which can result in broken seals at the welds and seams of the two halves of the fuel tank.
U.S. Pat. No. 5,441,781 teaches a multi-layer plastic fuel tank comprising (A) a gas barrier layer having on at least one side thereof (B) an adhesive layer and further thereon (C) a high-density polyethylene layer, the gas barrier layer (A) comprising a resin having gas barrier properties, the adhesive layer (B) comprising a resin having adhesiveness to both of the gas barrier layer (A) and the high-density polyethylene layer (C), the high-density polyethylene layer (C) comprising high-density polyethylene having an intrinsic viscosity of from 2 to 6 dl/g, a density of from 0.940 to 0.970 g/cm3, and a zero shear viscosity of from 2.0×107 to 1.0×108 poise at 190° C.
The multi-layer plastic fuel tank according to U.S. Pat. No. 5,441,781 comprises gas barrier layer (A) having laminated on at least one side thereof high-density polyethylene layer (C) via adhesive layer (B).
Resins with gas barrier properties are used in gas barrier layer (A). Examples include a modified polyamide composition comprising a mixture of (1) an α,β-unsaturated carboxylic acid-modified ethylene-α-olefin copolymer prepared by grafting an α,β-unsaturated carboxylic acid or a derivative thereof to an ethylene-. α-olefin copolymer at a grafting ratio of from 0.05 to 1% by weight, preferably from 0.2 to 0.6% by weight, based on the ethylene-. α-olefin copolymer and (2) a polyamide. The ethylene-α-olefin copolymer, preferably has a degree of crystallinity of from 1 to 35%, more preferably from 1 to 30%, and a melt index of from 0.01 to 50 g/10 min, more preferably from 0.1 to 20 g/10 min. Examples of the α,β.-unsaturated carboxylic acid or a derivative thereof include monocarboxylic acids, such as acrylic acid and methacrylic acid, their derivatives, dicarboxylic acids, such as maleic acid, fumaric acid and citraconic acid, and their derivatives. Preferred examples of the α,β-unsaturated carboxylic acid or a derivative thereof include maleic anhydride.
Examples of the α-olefins in the ethylene-α-olefin copolymer (1) include propylene, butene-1, hexene-1, etc. The α-olefin is generally copolymerized with ethylene at a ratio of not more than 30% by weight, and preferably from 5 to 20% by weight, based on the total amount of the copolymer.
The polyamide (2) generally has a relative viscosity of from about 1 to 6. Examples of the polyamide include polyamides obtained by polycondensation of a diamine and a dicarboxylic acid, polyamide obtained by polycondensation of an aminocarboxylic acid, polyamide obtained by polycondensation of a lactam, and copolyamide thereof.
Examples of the diamine includes aliphatic, alicyclic or aromatic diamines, such as hexamethylenediamine, decamethylenediamine, dodecamethylenediamine, trimethylhexamethylenediamine, 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, bis(p-aminocyclohexylmethane), m-xylylenediamine, and p-xylylenediamine. Examples of the dicarboxylic acid includes aliphatic, alicyclic or aromatic dicarboxylic acids, such as adipic acid, suberic acid, sebacic acid, cyclohexanedicarboxylic acid, terephthalic acid, and isophthalic acid. Examples of the aminocarboxylic acid includes ε-aminocaproic acid and 11-aminoundecanoic acid. Examples of the lactam includes ε-caprolactam and .ε-laurolactam.
Specific examples of the polyamide include nylon 6, nylon 66, nylon 610, nylon 9, nylon 11, nylon 12, nylon 6/66, nylon 66/610, and nylon 6/11.
From the standpoint of moldability, a polyamide having a melting point of from 170 to 280° C., and particularly from 200 to 240° C., is typical. Nylon 6 is quite suitable for the use.
The α,β-unsaturated carboxylic acid-modified ethylene-α.-olefin copolymer is generally mixed with the polyamide in an amount of from 10 to 50 parts by weight, and preferably from 10 to 30 parts by weight, per 100 parts by weight of the polyamide.
Other solutions to increasing the barrier of fuel tanks can be found in U.S. Pat. No. 5,129,544 which teaches a laminate structure with a chemical resistant layer such as nylon 12 and Teflon. U.S. Pat. No. 5,547,096 teaches a fuel tank of an inner and outer shell where the outer shell is electroplated with successive layers of copper, nickel and chrome. U.S. Pat. No. 6,409,040 teaches injection molding two halves where the barrier layer is formed on the outer wall as a coat of paint.
There therefore exists, the need for a single composition which can be used as a monolayer or as a barrier layer in the multilayer structure which is impermeable to fuel vapors and has the necessary mechanical strength to function as a fuel container.